In a charged particle beam system, a focusing column is typically used to focus a charged particle beam onto the surface of a target to be imaged or processed using the beam. A charged particle beam system can be, for example, an electron microscope or a focused ion beam system. To form an image of the target using scanning ion microscopy or scanning electron microscopy, the beam is deflected across the target surface, usually in a raster pattern. Due to the impact of the charged particle beam with the target, secondary particles are emitted and may be collected to form an imaging signal. The primary charged particle beam, that is, the beam that impacts the target, can be an electron beam or a focused ion beam.
An electron beam will generally stimulate the emission of secondary electrons from a target. A focused ion beam will stimulate the emission from a target of both secondary electrons and secondary ions, mostly positively-charged. Secondary particles are comprised of either secondary electrons, secondary ions, or a combination of secondary electrons and secondary ions. Secondary particles are detected by a detector, such as a micro-channel plate (“MCP”) also sometimes referred to as a “multi-channel plate,” a scintillator photomultiplier (also known as an Everhardt-Thornley or “ET” detector), or a semiconductor detector.
An MCP detector has a large number of small channels that are impacted by the secondary electrons from the target. Each channel operates independently of the others, amplifying the incoming secondary particles by a process that cascades the multiplication of the secondary particles within each channel. This amplified current is then collected on one or more anodes positioned on the far side of the MCP (i.e., the opposite side from the side receiving the input signal current). Often, to avoid “ion feedback,” that is, gas molecules in the MCP ionized by the electrons returning to impact the target, a two-stage structure is employed in which the channels in the first stage have a different angle than those in the second-stage, thereby eliminating “line-of-sight” travel of positive ions from the exit back to the entrance of the MCP (the so-called “chevron” configuration).
In a scintillator-photomultiplier detector, secondary electrons impact a scintillator, which gives off photons of light. A photomultiplier tube then converts the photons back to electrons, which are then amplified in a cascade process. In a solid state collector, each secondary electron is amplified by the creation of multiple electron-hole pairs in a semiconductor.
The detectors can have an annular shape concentric with the optical axis of the charged particle beam and a hole in the center to pass the particle beam. Alternatively, the detector can be “off axis.” An electron detector is characterized by a collection efficiency, i.e., the fraction of emitted secondary particles from the target which are collected by the detector.
FIG. 1 shows schematically a part of a prior art charged particle beam column 100. Charged particle column 100 includes a lens 104 that focuses a charged particle beam 106 onto the surface of a target 108. Due to the impact of charged particle beam 106 with target 108, secondary particles 116 are emitted from target 108. For the case where charged particle beam 106 is an electron beam, these secondary particles will be secondary electrons. For the case where charged particle beam 106 is a focused ion beam (FIB), both secondary electrons and secondary ions (mostly positive) may be emitted from the target 108. Generally, the angular intensity of the secondary particle emissions (in the case of a normally-incident primary charged particle beam 106), tends to follow a Lambert, or cosine distribution. The angular distribution of particles is concentrated around an axis perpendicular to the surface of target 108.
A microchannel plate detector 120 typically comprises two annular plates 122a and 122b, with the internal passages 124 of the two plates slanted in the opposite direction. Some detectors may include a single plate, while other detectors may include three or more plates. A grid 126 positioned between plate 122a and target 108 is maintained at a constant positive voltage to attract electrons from the target and to prevent positive secondary ions from entering and damaging the plates 122a and 122b. A collection anode 130 is positioned behind plate 122b to collect the amplified electron signal.
In one application called “circuit edit,” a charged particle system allows product designers to reroute conductive pathways of an integrated circuit and test the modified circuit in hours, rather than the weeks or months that would be required to generate new masks and process new wafers. Fewer, shorter modification and test cycles allow manufacturers to ramp new processes to faster, profitable, high volume yields, and be first to market with premium priced new products.
Circuit edit can involve milling a hole using the focused ion beam to sever a buried conductor or to deposit conductive material in order to create a new conductor that connects components. Circuit edit often requires producing high aspect ratio holes, that is, a hole that is deeper than it is wide. When milling a hole to connect two circuit layers or to sever a buried conductor, the operator must determine the correct time to stop milling to avoid milling past the desired layer. Determining when to stop, referred to as “endpointing,” often relies on observing the milling process using secondary particles. It is difficult to form an image of the bottom of a high aspect ratio hole because the few secondary particles that escape the hole do not impact the detector, thus making it difficult to determine when the desired hole depth is achieved.